This invention provides a lightning protection device for structures, civil and industrial buildings, plants or the like. It includes an atmospheric electrode which is insulated from the ground and means suitable for earthing or grounding the electrode when the potential difference, which is associated with the electric field at ground, exceeds a pre-determined value.
The inventive device develops a preventive action by interacting during the downward phase of the discharge. Therefore, it can stop the advancing and development of the discharge and avoid the formation of return strokes, which is the most dangerous effect of lightning.
First of all, it is opportune to summarize the phenomena which lead to the formation of lightning. These phenomena are not yet fully explained. During storms we have inside the clouds, the formation of a storm nucleus characterized by a greater concentration of electric charges, generally positive in the upper layers and negative in the lower zones, at a distance from ground of about 2-4 km.
Therefore, between the earth's surface and clouds a strong electric field is produced. The potential difference can reach 100,000,000 volts.
When the potential difference succeeds in overcoming the dielectric resistence of air, lighting is produced, generally beginning with a leader stroke which moves in zig-zagging fashion toward the earth and carries a high quantity of current.
During the final downward phase the electric field at ground reaches such high values that a certain quantity of opposite sign charges (streamer) leaves the ground to meet the downward stroke. A conductive channel is thus produced through which the leader stroke charges are scattered to ground. A violent return stroke then occurs with development of heat and luminous energy. This is the most dangerous effect of lightning.
The described phenomena are widely illustrated in several scientific publications. See in particular:
M. A. Uman "Lightning" Mc Graw-Hill (1969). PA1 R. H. Golde (Edited) "Lightning: Physics of Lightning and Lightning Protection" Academic Press (1977). PA1 J. A. Chalmers "Atmospheric Electricy" Pergamon Press, Oxford (1967). PA1 H. Baatz "Uberspannungen in Energieversorgungsnetzen" Springer-Verlag (1956). PA1 T. Suzuki, F. Miyake, I. Kishizima "Study on experimental simulation of lightning strokes" IEEE-PAS April 1981.
The presently employed devices for lightning protection include one or more catching elements which are connected to earth by means of an opportunely sized conductor so as to constitute a preferential way for the strokes to be scattered at ground. These catching elements generally consist of pointed metallic rods or catching nets which are fixed to the upper part of the structures to be protected and connected to buried earth elements.
Such lightning rods present however several disadvantages: In the first place they offer a limited protection because they are not capable of bearing strokes of greater intensity than that for which they have been fitted.
Moreover, they are expensive because of their particular connection to earth, and require periodical maintenance.
More recently, some experiments were made with lightning rods provided with radioactive points in order to make the lightning rods more efficient. The hoped for results, however have not been obtained in this case either. Moreover, radio-active points can represent a danger to the extent that they have been forbidden in some countries.
The Italian Patent No. 767,809 in the name of De Bernardi describes means to protect TV antennas from lightning. To this end circular shielding elements are provided near the antenna dipole, to form a barrier in the vicinity to the parts most exposed to the danger of strokes.
According to the teachings of such patent, the circular shielding elements deviate the lightning strokes and oppose their effects. In other publications by the same author reference is made to protection devices the aim of which is to disperse the stroke over a wide surface so as minimize the destroying effects thereof, or to the exploitation of the high frequency electromagnetic fields to produce a shielding barrier which can deviate the lightning. However, no relevant teachings are given, nor arguments are brought in support of these theories and the working of the relative devices is not described either.